Radio Communications in the Digital Age - VHF-UHF

First Printing, June 2000Copyright © 2000By Harris CorporationAll rights reserved

Library of Congress Catalog Card Number: 00 132465Harris Corporation, RF Communications DivisionRadio Communications in the Digital AgeVolume Two: VHF/UHF Technology

Printed in USA6/00 RO 10KB2008© Harris Corporation

All Harris RF Communications products and systems included herein are trademarks of Harris Corporation.

RADIO COMMUNICATIONSIN THE DIGITAL AGE

VOLUME TWO:VHF/UHF TECHNOLOGY

With gratitude to Hal Herrick for his dedication and fortitude inwriting this handbook .

INTRODUCTION 1

CHAPTER 1 PRINCIPLES OF RADIOCOMMUNICATIONS 5

CHAPTER 2 VHF/UHF RADIO PROPAGATION 20

CHAPTER 3 ELEMENTS IN A VHF/UHFRADIO SYSTEM 30

CHAPTER 4 NOISE ANDINTERFERENCE 49

CHAPTER 5 DATA COMMUNICATIONVIA VHF/UHF RADIO 53

CHAPTER 6 UHF SATCOM 69

TABLE OF CONTENTS

CHAPTER 7 SECURINGCOMMUNICATIONS 77

CHAPTER 8 SYSTEMS ANDAPPLICATIONS 84

CHAPTER 9 FUTURE DIRECTIONS 95

GLOSSARY 98

Note: Throughout this handbook, technical terms and acronyms shown in italics are defined in the Glossary.

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This is Volume 2 of an introductory series on radio communicationsTechnology. This volume covers very high frequency (VHF), ultra highfrequency (UHF), and Satellite Communications (SATCOM) technologies,

as well as the modern digital coding and modulation capabilities that optimizeinformation output and security.

The Communicator’s Tool Kit

A carpenter relies upon an assortment of chisels, drills, and hammers to do the job. Each type of tool is suited for a particular task. Likewise, the modern communications system designer makes use of HF, VHF, UHF, andSATCOM tools, and capitalizes on the unique capabilities that each brings to meet the requirements. Summaries of the most prominent capabilities of each radio frequency band are given below. Subsequent chapters will describe them in more detail.

HF: Around the Corner to Around the World

Before SATCOM technology existed, HF radios were the only means of communicating to ships at sea. The fact that HF can communicate beyondthe horizon makes it an indispensable tool for long distance ship-to-ship and ship-to-shore messages. Likewise, before the days of the transatlantic cables, HF (or short wave) radios were the only way to talk between continents. Today they are still used to share the overall burdens of long distance communications.

But the unique virtue of HF radio has also created some challenges.Worldwide radio transmissions are easy to intercept and the HF spectrumis complicated by signals emanating from the many individual transmitterslocated around the world. Special techniques must be implemented in theradios to take advantage of the radio’s long range, while still preservingthe clarity of the channel and reduce interception.

INTRODUCTION

Encryption reduces unfriendly utilization of intercepted signals and sophisticated coding schemes helps fight through clutter, but these techniques can reduce throughput (compared to that of a clear channel).Nevertheless, HF radios still play an indispensable role in the communicator’stool kit. HF manpack radios with various antenna options can cover a practical range of from “around the corner” to “around the world.”

Although some long range communications are now transmitted via satellite, HF still has the advantage of not requiring (or relying on) any infrastructure.

VHF: Man to Man

The VHF band was an early choice for manpack radios used by groundtroops to communicate within a local (five-mile or so) area. Antennas and selective-tuning components of VHF radios are very much smallerthan their HF counterparts.

Advances in the semiconductor industry have also increased the efficiencyof VHF radios because batteries are smaller, lighter, and longer-lived thanthose required in the past.

Unlike HF, VHF transmissions lack the ability for ionospheric bounce andare limited to line-of-sight (LOS) communication. This reduces radio emis-sion clutter throughout an extended battlefield and limits the vulnerabilityto unfriendly interception.

The wider channel bandwidth capabilities of VHF radios increase the efficiency of coding and encryption schemes and allow greater datathroughput than that of HF radios. Wider bandwidth and limited range make these radios ideal for squad-to-squad communications.

UHF: Ground to Air for Close Support

UHF tuning elements and antennas are even smaller than those of VHFand are much easier to mount on supersonic fighter aircraft, making UHFan ideal choice for ground-to-air communications. Like VHF radios, UHFradios share the advantages of being line of sight and having wide band-width. Modern military forces now prefer the UHF spectrum for ground-to-air communications.

3

SATCOM: Hails to (and from) the Chief

It is essential for front line units to communicate with the command centers that are sometimes hundreds, if not thousands, of miles away.With the advent of military satellites, SATCOM technology can complement HF equipment.

Although LOS is typically five miles or less along the ground betweenmanpack radios, the vertical LOS range of a UHF signal is tens of thousands of miles. This enables UHF radios to reach orbiting militarysatellites that are designed to retransmit the signal back to earth. The retransmitted signal covers a huge footprint and is ideal for long rangecommunications.

The highly directional antennas pointed up at the sky that are used withTactical Satellite (TACSAT) radios, reduce ground radiation of front lineTACSAT traffic. This makes SATCOM traffic much more difficult to intercept than that from HF radios.

Although it is true that the enemy can receive a downlink from a satellite,encryption denies access to the data and the source of the emanationsgives no clue as to the location of either the source or the destination of the data path.

The Multiband Radio

In this age of specialization a large, conventional military force has theability to carry an assortment of radios, each carefully designed for a specific purpose. But the situation of a small Special Forces combat teamis very different. Although they need to have access to all of the availablemilitary communication channels, they can’t afford the luxury (or theweight) of carrying the required multitude of radios. Hence, the multiband radio.

Similarly, installation space constraints in vehicles, shelters, small boats,etc., are being addressed with multiband radios.

Just as the small multipurpose pocket tool that folds open to display aknife, screw driver, pliers, and can opener can serve as an emergency toolbox, the multiband manpack radio is designed for the multipurpose needsof Special Forces. Some multiband radios also provide a satellite link allowing them to extend their range further.

Putting it all together

Subsequent chapters in this handbook develop the basic principles andoperating modes of the radios mentioned in this introduction. Operatingcharacteristics of each radio frequency band will be described and compared with respect to performance and application.

But there is a lot more than frequency band that defines the performanceof a radio. The following pages also touch upon the world of exotic waveforms that punch through noise, defy attempts at interception, andprovide data high data rates that previously were considered impossible.

Stay tuned!

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D eveloping an understanding of radio communications begins with thecomprehension of basic electromagnetic radiation.

Radio waves belong to the electromagnetic radiation family, which includes x-ray, ultraviolet, and visible light — forms of energy we use every day. Muchlike the gentle waves that form when a stone is tossed into a still lake, radiosignals radiate outward, or propagate, from a transmitting antenna.However, unlike water waves, radio waves propagate at the speed of light.

We characterize a radio wave in terms of its amplitude, frequency, and wavelength (Figure 1-1).

Radio wave amplitude, or strength, can be visualized as its height — the distance between its peak and its lowest point. Amplitude, which ismeasured in volts, is usually expressed in terms of an average value calledroot-mean-square, or RMS.

The frequency of a radio wave is the number of repetitions or cycles itcompletes in a given period of time. Frequency is measured in hertz (Hz); onehertz equals one cycle per second. Thousands of hertz are expressed as kilohertz(kHz), and millions of hertz as megahertz (MHz). You would typically see afrequency of 2,182,000 hertz, for example, written as 2,182 kHz or 2.182 MHz.

Radio wavelength is the distance between crests of a wave. The product ofwavelength and frequency is a constant that is equal to the speed of propaga-tion. Thus, as the frequency increases, wavelength decreases, and vice versa.

CHAPTER

1PRINCIPLES OF RADIO COMMUNICATIONS

87

Since radio waves propagate at the speed of light (300 million meters per second), you can easily determine the wavelength in meters for anyfrequency by dividing 300 by the frequency in megahertz. So, the wave-length of a 10-MHz wave is 30 meters, determined by dividing 300 by 10.

The Radio Frequency Spectrum

In the radio frequency spectrum (Figure 1-2), the usable frequency range forradio waves extends from about 20 kHz (just above sound waves) to above30,000 MHz. A wavelength at 20 kHz is 15 kilometers long. At 30,000 MHz,the wavelength is only 1 centimeter.

The High Frequency (HF) Band

The HF band is defined as the frequency range of 3 to 30 MHz. In practice,most HF radios use the spectrum from 1.6 to 30 MHz. Most long-haulcommunications in this band take place between 4 and 18 MHz. Higherfrequencies (18 to 30 MHz) may also be available from time to time,depending on ionospheric conditions and the time of day (see Volume One,HF Technology).

Very High Frequency (VHF) Band

The VHF frequency band is defined as the frequency range from 30 to 300MHz. From the previous discussion about the relationship between frequencyand wavelength, it should be noted that VHF wavelengths vary from 10-metersat the low end to one meter at the high end. This means that the size ofantennas and tuning components used in VHF radio are much smaller andlighter than those of HF radios. This is a big advantage for manpack radios.We will also see in later chapters that the higher frequency and shorter wavelengths of VHF radios have a profound effect on radio range.

Ultra High Frequency (UHF) Band

The UHF band goes from 300 MHz to 2450 MHz, although TACSAT manpackUHF radios do not utilize frequencies above 512 MHz. The wavelengths associated with 300 to 512 MHz range from one meter to 0.58 meters (58 centimeters). The very small antennas required for these wavelengthsmake them ideal for use on high-speed aircraft.

Frequency Allocations

Within the HF spectrum, groups of frequencies are allocated to specific radioservices — aviation, maritime, military, government, broadcast, or amateur

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(Figure 1-3). Frequencies are further regulated according to transmission type:emergency, broadcast, voice, Morse code, facsimile, and data. Internationaltreaty and national licensing authorities govern frequency allocations.

Frequencies within the VHF/UHF bands are similarly allocated (Figure 1-4).

Modulation

The allocation of a frequency is just the beginning of radio communications.By itself, a radio wave conveys no information. It’s simply a rhythmic stream of continuous waves (CW).

When we modulate radio waves to carry information, we refer to them ascarriers. To convey information, a carrier must be varied so that its properties— its amplitude, frequency, or phase (the measurement of a complete wavecycle) — are changed, or modulated, by the information signal.

The simplest method of modulating a carrier is by turning it on and off bymeans of a telegraph key. In the early days of radio, On-Off keying, usingMorse code, was the only method of conveying wireless messages.

Today’s common methods for radio communications include amplitudemodulation (AM), which varies the strength of the carrier in direct proportionto changes in the intensity of a source such as the human voice (Figure 1-5a).In other words, information is contained in amplitude variations.

The AM process creates a carrier and a pair of duplicate sidebands — nearbyfrequencies above and below the carrier (Figure 1-5b). AM is a relatively ineffi-cient form of modulation, since the carrier must be continually generated. Themajority of the power in an AM signal is consumed by the carrier that carriesno information, with the rest going to the information-carrying sidebands.

In a more efficient technique, single sideband (SSB), the carrier and one of the sidebands are suppressed (Figure 1-5c). Only the remaining sideband,upper (USB) or lower (LSB), is transmitted. An SSB signal needs only half thebandwidth of an AM signal and is produced only when a modulating signal is present. Thus, SSB systems are more efficient both in the use of the spec-trum, which must accommodate many users, and of transmitter power. Allthe transmitted power goes into the information-carrying sideband.

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One variation on this scheme, often used by military and commercial communicators, is amplitude modulation equivalent (AME), in which a carrier at a reduced level is transmitted with the sideband. AME lets one use a relatively simple receiver to detect the signal. Another important variation is independent sideband (ISB), in which both an upper and lower sideband,each carrying different information, is transmitted. This way one sideband can carry a data signal and the other can carry a voice signal.

Frequency modulation (FM) is a technique in which the carrier’s frequencyvaries in response to changes in the modulating signal (Figure 1-5d). For avariety of technical reasons, conventional FM generally produces a cleanersignal than AM, but uses much more bandwidth. Narrowband FM, which is sometimes used in HF radio, provides an improvement in bandwidth utilization, but only at the cost of signal quality.

It is in the UHF and VHF bands that FM comes into its own. Remember thatthe HF band is generally defined as occupying the spectrum from 1.6 MHz to 30 MHz. This is a span of only 28.4 MHz. The VHF band covers the spanof from 30 MHz to 300 MHz, which is a span of 270 MHz; nearly 10 times the span of HF. This extra room means that a channel bandwidth of 25 kHz is used to achieve high signal quality.

Other schemes support the transmission of data over radio channels,including shifting the frequency or phase of the signal. We will cover thesetechniques in Chapter 5.

Radio Wave Propagation

Propagation describes how radio signals radiate outward from a transmittingsource. The action is simple to imagine for radio waves that travel in a straightline (picture that stone tossed into the still lake). The true path radio wavestake, however, is often more complex.

There are two basic modes of propagation: ground waves and sky waves. As their names imply, ground waves travel along the surface of the earth,while sky waves “bounce” back to earth. Figure 1-6 shows the different propagation paths for radio waves.

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Ground waves consist of three components: surface waves, direct waves, and ground-reflected waves. Surface waves travel along the surface of theearth, reaching beyond the horizon. Eventually, the earth absorbs surfacewave energy. The frequency and conductivity of the surface over which thewaves travel largely determine the effective range of surface waves.Absorption increases with frequency.

Transmitted radio signals, which use a carrier traveling as a surface wave, aredependent on transmitter power, receiver sensitivity, antenna characteristics,and the type of path traveled. For a given complement of equipment, therange may extend from 200 to 250 miles over a conductive, all-sea-waterpath. Over arid, rocky, non-conductive terrain, however, the range may drop to less than 20 miles, even with the same equipment.

Direct waves travel in a straight line, becoming weaker as distance increases.They may be bent, or refracted, by the atmosphere, which extends theiruseful range slightly beyond the horizon. Transmitting and receiving antennasmust be able to “see” each other for communications to take place, soantenna height is critical in determining range. Because of this, direct wavesare sometimes known as line-of-sight (LOS) waves. This is the primary modeof propagation for VHF and UHF radio waves.

Ground-reflected waves are the portion of the propagated wave that is re-flected from the surface of the earth between the transmitter and receiver.

Sky waves make beyond line-of-sight (BLOS) communications possible. Atfrequencies below 30 MHz, radio waves are refracted (or bent), returning toearth hundreds or thousands of miles away. Depending on frequency, time of day, and atmospheric conditions, a signal can bounce several times before reaching a receiver.

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SUMMARY

■ Radio signals radiate outward, or propagate, from a transmittingantenna at the speed of light.

■ Radio frequency is expressed in terms of hertz (cycles per second),kilohertz (thousands of hertz), or megahertz (millions of hertz).

■ Frequency determines the length of a radio wave; lower frequencies have longer wavelengths and higher frequencies have shorterwavelengths.

■ Long-range radio communications beyond line of sight (BLOS) take place in the high-frequency (HF) range of 1.6 to 30 MHz. Different portions of this band are allocated to specific radio services under international agreement.

■ Short-range radio communications (LOS) communications can take place at all radio frequencies, but that task is most often given to the VHF and UHF radio bands.

■ Using sky waves can be tricky, since the ionosphere is constantlychanging. Sky wave propagation is generally not available in theVHF and UHF frequency bands.

■ Modulation is the process whereby the phase, amplitude, or frequency of a carrier signal is modified to convey information.

CHAPTER

2VHF/UHF RADIO PROPAGATION

Chapter 1 explains that HF, VHF, and UHF radio waves have differentpropagation characteristics. VHF and UHF radio frequencies propa-gate principally along LOS paths. On the other hand, HF waves, those

below 30 MHz, can also reflect off the ionosphere and then travel back toearth. These sky waves, as they are called, give rise to one of the most important attributes of HF radio and that is beyond the line of sight (BLOS)communication. Volume One, “HF Technology”, of this series “RadioCommunications in the Digital Age” provides a detailed description of HFpropagation. This chapter deals primarily with LOS propagation characteris-tics of the VHF and UHF frequency bands.

While many HF propagation characteristics are associated with the ionos-phere and wave reflections from it, the effects of local area topography and conditions in the lower atmosphere mostly govern VHF and UHF propa-gation. Similarly, ground wave propagation is a very important mode of HFwave propagation, but at frequencies above 30 MHz, ground waves areabsorbed almost immediately and have a negligible beneficial impact.

Frequencies in the VHF and UHF bands usually penetrate the ionosphere andspeed out into space. That means that reflection off the ionosphere can notbe used to reliably extend communications range of these frequencies. Forthe most part, the transmitting and receiving antennas must have a fairlyunobstructed path between them for communication to take place, hencethe term line-of-sight (LOS).

Height Matters for LOS Range

The visible horizon observed at approximately five feet above a flat surface of earth is less than 2.7 miles away (Figure 2-1a). This is approximately themaximum LOS radio range from a manpack radio on the back of a standingman to another manpack radio that is lying on the ground (Figure 2-1b).

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Figure 2-1b shows that if the receiving radio were elevated to the back of astanding man, this maximum distance would be doubled. In this case the LOSdistance would be 5.4 miles. But, if the second man was standing beyond thisdistance, say at 7 miles from the transmitting radio, the shadowing effects ofthe earth’s curvature would prevent the second man from receiving the radiowave. In this case, 7 miles is BLOS and is not within reach of VHF or UHFradios in these positions.

It is clear that the elevation of both the transmitting and receiving antennas iscrucially important. For example if the receiving antenna were mounted on a26-foot tower, the total LOS distance would be increased to 9 miles. Of courseif the radiomen were both located on the tops of mountains, the LOS rangemight be as much as from 50 to hundred miles.

For ground-to-air UHF communications, the aircraft can be 100 miles away or more and still maintain contact.

Transmit Power and Radio Range

For HF radio communications, transmit power is an important item. For verylong distances, particularly for both skywave and ground wave propagation,every mile of distance attenuates (decreases) the signal. For most systems,when doubling the distance, the radiated signal is divided by four! Therefore,transmit power is often the limiting range factor. It is common to see 500-wattand 1-kW HF transmitters in vehicular or shipboard HF applications, and 10-kWor greater for HF fixed station broadcast sites.

VHF and UHF waves are also attenuated with every mile of distance. However,for tactical manpack applications, it is most often the shadowing effects ofirregular terrain, buildings, and other objects that limit the effective rangeand not transmit power.

Many manpack radios have two power settings: 2 Watts and 5 to 10-Watts. The 2-Watt setting is often adequate and extends battery life when thispower level is selected. On the other hand, there are situations whereincreased power is beneficial. In urban areas where high radio frequencynoise is prevalent, higher power increases the signal-to-noise (SNR) ratioand improves reception. Also, modern high data rate modulation wave-forms require a high SNR to be effective.

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UHF ground-to-air communications benefit from higher power because thetypical range is 100-miles or more.

Lastly, although tactical manpack UHF SATCOM radios with only 18-Wattslocated in Europe can contact a satellite in an orbit 22,000 miles above theearth’s equator, communication is more reliable when higher power is used.

These higher power VHF and UHF radio sets are typically mounted in vehiclesor fixed stations with 50-watt power amplifiers to boost the power of themanpack transceiver.

VHF and UHF Radio Reception Behind Ridges

For the most part, ridges and hills form shadows of VHF and UHF radio waves.However, there is an important exception when it comes to very sharp ridgesor other kinds of abrupt barriers. This is caused by a phenomenon known asDiffraction (Figure 2-2). When a VHF or UHF wave comes to a sharp edge, aportion of the wave bends around the edge and continues propagation as ifa very low power radio was placed at the top of the ridge. It is important thatthe ridge be relatively sharp. A well-rounded hill or the curvature of the earthis not sufficient to cause this effect. This effect is important in a battle fieldsituation where a soldier must seek shelter behind a ridge.

Reflections and Multipath Distortion

VHF and UHF waves can be reflected off of dense surfaces like rocks orconductive earth, just like a beam of light can be reflected off a wall or aceiling. Sometimes several paths exist between a transmitting and receivingantenna (Figure 2-3). In this figure there is a direct LOS path between tworadios, but there is also a reflected path from the bottom of a valleybetween them.

It is clear that these two paths are of different length, and that the directpath is the shorter of the two. Since radio waves travel at a constant velocity,the direct path wave arrives at the receiver before the reflected path. Thismeans that the same broadcast information reaches the receiver at twodifferent times.

The effect of this is much like echoes that one hears in an acoustically poorroom. If the echoes are close enough to each other, it is hard to understandwhat is being said. In radio terminology, this is called multipath distortion.Although it is annoying with voice communications, it is devastating to high

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data rate digital communication. A subsequent chapter will discuss some of the ingenious ways that have been devised to minimize the effects of this type of distortion.

“Picket fencing” is a form of multipathing common to vehicular mountedradios. It is prevalent with VHF and UHF. The higher the frequency, the morepronounced the effect is. It is usually caused by interference or reflections ofsignals from man-made objects such as buildings, houses, and other structures.

These objects cause constructive and destructive fields (or strengthened andweakened signals) so that when a vehicle travels through these fields, it receivesalternately stronger and weaker signals. There is usually a “swishing” sound in the receiver, as the signals rapidly grow weaker, then stronger, then weaker again.

The signal peaks and nulls are a function of wavelength. A 450-MHz signalbeing received on board a vehicle travelling at 60 mph can “flutter” veryrapidly as the vehicle travels through the downtown area of a city. You canexperience the same phenomenon on the lower VHF bands, but the fluttersare not quite as rapid.

Sometimes this same effect is caused by signals of two stationary radiosreflecting off a moving aircraft above them.

Multipath within a Building

In tactical situations, manpack radios are frequently operated under cover inbuildings. VHF and UHF waves have trouble penetrating reinforced concreteexterior walls, but they pass through windows and light interior wall partitionswith comparative ease.

Figure 2-4 shows a receiver in a room of a building with a transmitter locatedoutside. In this case, there are three paths from the transmitter to the receiver,and none of them are direct.

Path 1 passes through the window nearest the receiver location, and isdiffracted around the sharp edge of the window frame to the receiver.Likewise, path 2 just misses having a direct path to the receiver. It isdiffracted slightly by the window frame nearest the transmitter and thenpasses through an interior wall on the way to the receiver. Path 3 goesthrough a window and an interior wall before striking an outside wall of the building and then reflecting back to the receiver.

27 28

Each of these paths has a different distance and, therefore, can cause multipath distortion. Frequently just moving the receiver a few feet in somedirection will avoid one or more of the available paths and the reception of the signal may be greatly improved.

VHF and UHF Wave Ducting

The suggested limits on LOS range are sometimes exceeded in practice. One of the principal reasons for this is an effect called “ducting.” VHF and UHF waves traveling through the atmosphere travel slightly slower than they do in free space, and that is because the density of air slows them down. The denser the air, the slower the wave speed through it.

Under normal conditions, the density of air is the greatest at the surface of the earth and gradually reduces in density with altitude. Under fair, dry, and moderate weather conditions; the slight variations in air density have negligible effects on the path of radio waves passing through it.

Frequently there are abrupt changes in air density due to weather frontspassing over an area or the heavy moisture burden of rain clouds. In such cases VHF and UHF can bend or duct between air layers of different densities.Sometimes this ducting bends the radio waves downward so that the radiowaves tend to follow the curvature of the earth. In such cases the LOS range is considerably greater than the optical LOS range.

This type of wave propagation is impossible to predict; it is not practical to plan on it for range improvement. However, when ducting conditions exit,they generally do so for hours at a time.

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29 30

SUMMARY

■ HF propagation can be LOS through ground waves or direct waves and BLOS though the use of sky waves.

■ VHF and UHF frequencies cannot make use of skywave or ground wave propagation and depend almost exclusively on the direct wave. This restricts their use to LOS communications.

■ Radio wave propagation at VHF and UHF frequencies are primarilyaffected by local area topography (hills and valleys) and atmosphericconditions.

■ VHF and UHF range is usually limited by physical wave shadowing of obstructions such as buildings and mountains.

■ Diffraction of VHF and UHF waves can bend them around sharp edges such as window frames or sharp ridges.

■ Multipath distortion is caused by waves arriving at a receiver from more than one path.

■ LOS range is greatly improved with increased height of either(or both) transmit or receive antennas.

■ Ducting caused by certain weather conditions can sometimesincrease the range of VHF and UHF waves.

CHAPTER

3

N ow that you have an overview of how radio waves propagate,let’s take a look at how they are generated. The primary compo-nents in a VHF or UHF radio system fall into three groups: trans-

mitters, receivers, and antennas. In most modern radio tactical sets, thetransmitter and receiver are contained in a single unit called a transceiver.This chapter presents an overview of these radio system elements.

The Anatomy of a Multiband VHF/UHF Transceiver

In times past, a tactical transceiver was restricted to a single band. That is to say that a separate radio was required for HF, VHF, and UHF service.With the increased requirement for greater troop mobility there is enormous pressure to compress all of these separate radios into a single multiband radio. Thanks to electronics miniaturization; multiband, multimode radio systems are a reality.

Currently, transceivers capable of VHF, UHF, and Tactical Satellite (TACSAT)service are common. A combined HF/VHF/UHF and TACSAT transceiver isthe latest innovation in this area.

The simplest transceiver must generate a modulated signal to the antennaand to receive a signal from an antenna, demodulate it, and feed the information to a headset, computer, or some other human or machine interface. A multiband transceiver must perform these functions for eachof its frequency bands (Figure 3-1).

Most functions of the multiband transceiver are common to all frequencybands; however, the electronic means to accomplish these functions

ELEMENTS IN A VHF/UHF RADIO SYSTEM

differ depending upon the operating frequency band. Thus those functions that are associated with VHF transmit and receive frequenciesmust be grouped separately from those that perform that function for the UHF band. That is why most of the RF portions of the transceiver must be duplicated for each band, as shown in Figure 3-1.

Transmit Path Begins with the Digital Signal Processor

The transmitted voice or data information is applied to a common block in a multiband transceiver called the Digital Signal Processor (DSP). The DSP is actually a powerful but miniature computer that turns the input information into a digital form that is manipulated within the computer.

The functions performed by the DSP include audio bandwidth filtering,voice digitization, encryption, and modulation. The output of the DSP is actually a Low Frequency (LF) modulated carrier that is an exact replica ofwhat is to be transmitted, except for its frequency. This signal is referred to as being at an Intermediate Frequency (IF).

UHF Frequency Up-Conversion, and Frequency Synthesizer

If a UHF frequency is selected, the IF signal at the output of the DSP is applied to the UHF up-converter circuits. Another block of circuits, called a frequency synthesizer creates the various signals that are required by the up-converter to create the desired UHF output frequency.

Power Amplifier and Transmit Filters

The up-converted signal is then applied to a wideband power amplifierwhich covers the entire transmit band selected. In this case it is the UHFband and the amplifier that handles signals from 90 to 512 MHz. The signal power output of this amplifier is typically operator selected from 1 to 10 watts.

Following the power amplifier is a group of switched low pass filters that“clean up” its output. These remove noise, spurious signals, and harmonicsgenerated by other transmitter circuits including frequency harmonics generated by the power amplifier. This process reduces interference withadjacent communications channels.

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The output of the UHF low pass filters is applied through a Transmit/Receive (TX/RX) switch (shown in Figure 3-1 in the TX position) to the UHF antenna port of the transceiver. UHF antennas have a 50-ohm input impedance.

Receive Path Begins with Switched Bandpass Filters

A receive UHF signal is applied by the antenna to the antenna port, andthen through the TX/RX switch to a group of switched bandpass filters. The purpose of these filters is to remove signals above and below the desired signal.

RF Amplifiers and Down-Converter

The filtered input signals are applied to several radio frequency amplifierstages (shown as one block in Figure 3-1). The typical input signal has a signal strength in the micro-watt range (one millionth of a watt). The RFamplifiers boost this signal to the milli-watt range for further processing.

The next step in this process is to down-convert the signal to the LF IF frequency used by the DSP block. Again, this is accomplished by the down-converter in conjunction with signals from the synthesizer. In modern radios, this process is performed in several separate amplificationand down conversion steps. It is shown in Figure 3-1 as occurring injust one step for simplicity.

DSP Demodulation and Decryption

The final steps in the receive process are performed by the DSP. Here the IF signals from the down-converter is demodulated and decrypted to form the base band signals (audio or data) that are used by the operator.

VHF Band Portion of the Transceiver

The VHF transmit and receive functions are similar to those of the UHFband except that they are performed by the VHF portions of the radio.However there is one additional function required in the VHF band andthat is antenna matching.

VHF Whip Antenna Matching

The whip antenna frequently used with a VHF manpack radio does not present a 50-ohm impedance to the radio over the 30 to 90 MHz band. In order to maximize the power radiated from this type of antenna, a series of switched matching circuits are used in the transmit path following theswitched low pass filters. The correct matching network is selected auto-matically by the frequency selector switch on the transceiver front panel.

50-Watt Multiband Transceiver Group

It is common for radios used in vehicles and in fixed stations to requirehigher power than the tactical manpack transceiver can deliver on its own.In these applications, the manpack transceiver is attached to a mountingbase that includes power amplifiers and some additional antenna ports(Figure 3-2).

Power Amplifiers

A multiband vehicular adapter is likely to have two or more power amplifiers that are tailored to the frequency ports of the manpack transceiver. Figure 3-2 shows a vehicular adapter with both VHF and UHFtransceiver ports. Each of these ports is associated with a power amplifierthat is capable of producing 50 watts of output power.

Each of these amplifiers has a receive bypass path which is selected by thetransceiver keyline. In the key-down transmit condition, the bypass is openand the signal is applied to the power amplifiers. However, in the receivecondition, the amplifiers are bypassed so that the signal from the antennaports can pass back to the receiver circuits in the manpack transceiver.

VHF Low, VHF High, UHF, and TACSAT Antenna Ports

Most multiband transceivers have two antenna ports, one for VHF and the other for UHF. In vehicular and fixed station installations it is commonto have antennas that are larger and more efficient than those used with a manpack alone. It is therefore, convenient to have four antenna ports.The first port is used for low band VHF over the 30 to 89.999 MHz range.But the UHF path is spread between three separate antenna ports, asshown in Figure 3-2.

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The output of the UHF amplifier is applied to a diplexer, which splits the UHFport into two frequency ranges, 90 to 224.999 MHz and 225 to 512 MHz.Each of these frequency outputs is applied to a corresponding antenna port.

The 225 to 512 MHz path is further divided by a relay switch into a UHFpath and a TACSAT path. This is because the TACSAT path requires a collocation filter, a Low Noise Receive Amplifier (LNA), and a separate antenna port. This additional filtering and amplification on the TACSAT path is useful because of the typically low level of received signal from thetactical satellite in orbit 22,300 miles above the equator. The collocationfilter is there to remove radio noise generated by vehicle ignition, motors,and other transmitters that would otherwise obscure the faint signals from the satellite.

The Antenna Group

The antenna is one of the most critical elements in a radio circuit. Here, we will look at typical antenna types and their applications.

Antenna Characteristics and Parameters

Some of the most commonly used terms to describe antennas are impedance, gain, radiation pattern, take-off angle, and polarization.

Every antenna has an input impedance that represents the load to be applied to the transmitter. This impedance depends upon many factorssuch as antenna design, frequency of operation, and location of the antenna with respect to surrounding objects.

The basic challenge in radio communications is finding ways to get themost power possible, where and when you need it, to generate and transmit signals. Most transmitters are designed to provide maximum output power and efficiency into a 50-ohm load. (Ohm is a unit of measurement of resistance.) Some antennas, such as log periodic antennas, can provide a 50-ohm load to the transmitter over a wide range of frequencies. These antennas can generally be connected directly to the transmitter. Other antennas, such as dipoles, whips, andlong-wire antennas, have impedances that vary widely with frequency and the surrounding environment.

HF applications use an antenna tuner or coupler. This device is inserted between the transmitter and antenna to modify the characteristics of the load presented to the transmitter so that maximum power may be transferred from the transmitter to the antenna.

For most VHF and UHF applications, the antennas have built-in broadband-matching units so separate antenna coupler units are generally not required.

Antenna Gain and Radiation Pattern

The gain of an antenna is a measure of its directivity — its ability to focusthe energy it radiates in a particular direction. The gain may be determinedby comparing the level of signal received from it against the level thatwould be received from an isotropic antenna, which radiates equally in all directions. Gain can be expressed in dBi; the higher this number, thegreater the directivity of the antenna. Transmitting antenna gain directly affects transmitter power requirements. If, for example, an omnidirectionalantenna were replaced by a directional antenna with a gain of 10 dBi, a100-watt transmitter would produce the same effective radiated power as a 1-kW transmitter and omnidirectional antenna.

In addition to gain, radio users must understand the radiation pattern of an antenna for optimal signal transmission. Radiation pattern is deter-mined by an antenna’s design and is strongly influenced by its location with respect to the ground. It may also be affected by its proximity tonearby objects such as buildings and trees. In most antennas, the pattern is not uniform, but is characterized by lobes (areas of strong radiation) andnulls (areas of weak radiation). These patterns are generally representedgraphically in terms of plots in the vertical and horizontal planes (Figure 3-3), which show antenna gain as a function of elevation angle (verticalpattern) and azimuth angle (horizontal plot). The radiation patterns are frequency dependent, so plots at different frequencies are required to fully characterize the radiation pattern of an antenna.

In determining communications range, it is important to factor in the take-off angle, which is the angle between the main lobe of an antennapattern and the horizontal plane of the transmitting antenna. For VHF and UHF applications, low take-off angles are generally used for LOS communications; high take-off angles are used for ground-to-air, close air support.

37 38

NOTE: Example shown is foran antenna pointingtoward the east.

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Vertical Whip Radiation PatternFigure 3-4.

39 40

The orientation of an antenna with respect to the ground determines itspolarization. Most VHF and UHF whips and center fed monopole antennasare vertically polarized.

A vertically polarized antenna produces low take-off angles. The maindrawback of vertical whip antennas is their sensitivity to ground conductivity and locally generated noise. Center fed monopoles avoid thesensitivity to ground conductivity and are preferred for vehicular mounts.

Horizontally polarized antennas, such as a 1/2-wave dipole, have high elevation angles. This type of antenna is particularly useful when the transmitter is near a forest or jungle. This allows the radiation to get above the trees rather than having them absorbed. Diffraction at the treetops tends to bend the radiation down so that it follows the treetops.For best results, the transmitting and receiving antennas should have the same polarization.

VHF Antennas

There are a countless variety of antennas used in VHF communication.We’ll focus here on some of the more common types.

The vertical whip antenna is frequently used since it is omnidirectional and has low take-off angles. It is vertically polarized. A typical vertical whipradiation pattern is shown in Figure 3-4. A reflector, consisting of a secondvertical whip, can add directivity to the radiation pattern of a whip.

Another useful type of antenna is the center fed 1/4 wave dipole, which is basically two lengths of wire fed at the center (Figure 3-5). This is a horizontally polarized antenna and is frequently used for vehicular andfixed station applications.

The radiation pattern can change dramatically as a function of its distanceabove the ground. Figure 3-6 shows the vertical radiation pattern of a horizontal dipole for several values of its height (in terms of transmittingwavelength) above the ground.

An inverted vee (sometimes called a “drooping dipole”) produces a combination of horizontal and vertical radiation with omnidirectional coverage. See Figure 3-7.

41 42

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Figure 3-6.

43 44

INVERTED VEE ANTENNA

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Horizontal Log Periodic AntennaFigure 3-8.

Figure 3-9a. Center Fed Dipole

Figure 3-9b. SATCOM Antenna

45 46

For fixed station use on high elevations (high hills or mountain tops) a logperiodic directional antenna can be used for very long LOS communicationsof 100 miles or more. See Figure 3.8

UHF and SATCOM Antennas

For most UHF manpack applications, the transceiver is mounted with ashort, stubby antenna that resembles a hot dog in shape. This antenna is used for relatively short LOS distances and its virtue is its small size.

For vehicular or shelter mounted applications, an effective general-purposeUHF antenna is a center-fed dipole (Figure 3-9a). This antenna looks like athick whip antenna. It is constructed within a fiberglass tube and consistsof a dipole mounted vertically within the tube along with its feed point. Itssignificant virtue is that it is relatively independent of the ground quality. Ithas a low take off angle, and it is vertically polarized. The center-fed dipoleantenna has a pattern similar to the whip pattern shown in Figure 3-4.There are center-fed dipoles designed for VHF frequencies as well.

The UHF Tactical Satellite (TACSAT) antenna has a unique inverted umbrellashape (Figure 3-9b). It produces a directed beam that must be pointed directly at the satellite in order to be effective.

For fixed station use, an elevated whip or center-fed dipole greatly increases the LOS range (Figure 3-9c). This antenna assembly consists of a mast and a vertical whip or dipole mounted above ground plane rods.Again, this antenna structure can be used for both VHF and UHF applica-tions with the proper selection of antenna and ground plane rod lengths.

Another popular UHF antenna used for fixed station use is the BiconicalAntenna shown in Figure 3-9d. An antenna of this type has been designedto cover the 100 to 400 MHz range. Its broadband capability makes it an excellent choice for wide band Transmission Security (TRANSEC) modes suchas frequency hopping. Refer to Chapter 7 for a discussion of TRANSEC.

The Biconical Antenna is usually mounted on a mast similar to the oneshown in Figure 3-9c.

SUMMARY

■ A radio system consists of a transceiver and an antenna group.

■ The transceiver provides both transmitting and receiving functions.

■ The transmit function consists of modulation, carrier generationfrequency translation, and power amplification.

■ The receive function consists of RF signal filtering, amplification,frequency down conversion, and demodulation.

■ Antenna selection is critical to successful VHF, UHF, and TACSATcommunications. Antenna types include vertical whips, center-fed dipoles, biconical antennas, directional log periodic arrays, andumbrella TACSAT antennas.

■ An antenna coupler matches the impedance of the antenna to that of the transmitter, transferring maximum power to the antenna.

■ The gain of an antenna is a measure of its directivity — its ability to focus the energy it radiates in a particular direction.

■ Antenna radiation patterns are characterized by nulls (areas of weak radiation) and lobes (areas of strong radiation).

47 48

Figure 3-9c. Elevated Whip Antenna

Figure 3-9d. VHF/UHF Biconical Antenna100 to 400 MHZ

Vertical Element

Antenna Base

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49 50

CHAPTER

4

W hile listening to the radio during a thunderstorm, you’re sureto have noticed interruptions or static at one time or another.Perhaps you heard the voice of a pilot communicating to a

control tower when you were listening to your favorite FM station. Theseare examples of interference that affect a receiver’s performance.Annoying as this may be while you’re trying to listen to music, noise andinterference can be hazardous in the world of communications, where amission’s success or failure depends on receiving and understanding thetransmitted message.

Sources of Noise

Receiver noise and interference come from both external and internal sources.Internal noise is created within the circuits of the receiver itself. Power suppliesand frequency synthesizers are prominent sources of noise within the radio. But some noise comes from thermal agitation of the molecules that comprise electronic components in the amplifier stage closest to the receiver antenna.

External noise comes to the receiver by way of the antenna from sources outside the radio and frequently exceed internal receiver noise.

Natural Sources of Noise

In the HF frequency band, lightning is the main atmospheric (natural) source of noise. Atmospheric noise is highest during the summer and greatest at night,especially in the l- to 5-MHz range. One of the advantages of the VHF and UHFbands is that they are above this large source of noise.

Another natural noise source is galactic or cosmic noise, generated in space. At 20 MHz (just below the VHF band), space noise is generally larger than that of internally generated noise. This noise tapers off so that at around 200 MHz,it is about equal to internal noise. At higher frequencies it is insignificant.

NOISE AND INTERFERENCE

Man-Made Noise

Power lines, computer equipment, and industrial and office machinery produceman-made noise, which can reach a receiver through radiation or by conductionthrough power cables. This type of man-made noise is called electromagneticinterference (EMI) and it is highest in urban areas. Grounding and shielding ofthe radio equipment and filtering of AC power input lines are techniques used by engineers to suppress EMI.

Unintentional Radio Interference

At any given time, thousands of radio transmitters compete for space on theradio spectrum above and below the VHF/UHF range of frequencies. Harmonicsof HF transmitters fall within the VHF band, and commercial FM stations andother wireless radio emissions fall directly within the VHF and UHF bands. Theradio spectrum is especially congested in Europe due to population density.

A major source of unintentional interference is the collocation of transmitters,receivers, and antennas. It’s a problem on ships where space limitations dictatethat several radio systems are located together.

Ways to reduce collocation interference include locating and carefully orientingantennas; using receivers that won’t overload on strong, undesired signals; andusing transmitters designed to minimize harmonics and other spurious emis-sions.

Intentional Interference

Deliberate interference, or jamming, results from transmitting on operatingfrequencies with the intent to disrupt communications. Jamming can be directedat a single channel or be wideband. It can be continuous (constant transmitting)or look-through (transmitting only when the signal to be jammed is present).

Modern military radio systems use spread-spectrum or frequency-hopping techniques to overcome jamming and reduce the probability of detection orinterception. Spread-spectrum techniques are techniques in which the modulated information is transmitted in a bandwidth considerably greater than the frequency content of the original information.

51 52

SUMMARY

■ Natural (atmospheric) and man-made sources cause noise andinterference. Power lines, computer terminals, and industrialmachinery are prominent causes of man-made noise.

■ Congestion of radio transmitters competing for limited radiospectrum causes interference.

■ Collocated transmitters interfere with nearby receivers.

■ Jamming, or deliberate interference, results from transmitting onoperating frequencies with the intent to disrupt communications.

■ Multipath interference can be considered another form of noise.

■ Proper antenna selection and advanced modulation techniques can reduce the effects of noise and interference.

Multipath Distortion

Signals from a transmitter reach the receiver via multiple paths and arrive atslightly different times (see Chapter 2). These multiple signals are as disruptive to communication as signal interference from other transmitters.

Signal Quality Measurement

Signal quality is indicated by signal-to-noise ratio (SNR), measured in decibels(dB). The higher the SNR, the better the signal quality. Every 3 dB of SNR corresponds to a ratio of two-to-one. Thus a 9 dB SNR means that the signal iseight times greater than the noise. A commonly considered SNR lower limit foradequate reception is 10 dB. This means that the signal has ten times as muchpower as the noise.

Reducing the Effect of Noise and Interference

Engineers use various techniques to combat noise and interference, including:(1) boosting the effective radiated power, (2) providing a means for optimizingoperating frequency, (3) choosing a suitable modulation scheme, (4) selectingthe appropriate antenna system, and (5) designing receivers that reject interfering signals.

53 54

states. For example, a switch is open or closed; a voltage is positive or negative, and so on.

A simple way to communicate binary data is to switch a circuit off and onin patterns that are interpreted at the other end of a link. This is essentiallywhat was done in the early days of telegraphy. Later schemes used a bit to select one of two possible states of the properties that characterize a carrier (modulated radio wave) — either frequency or amplitude. More sophisticated approaches allow the carrier to assume more than two states and hence to represent multiple bits.

Baud Rate

Data transmission speed is commonly measured in bits per second (bps).Sometimes the word baud is used synonymously with bps, although thetwo terms actually have different meanings. Baud is a unit of signalingspeed and is a measure of symbols per second that are being sent. A symbol may represent more than one bit.

The maximum baud rate supported by a radio channel depends on itsbandwidth — the greater the bandwidth, the greater the baud rate. Therate at which information is transmitted, the bit rate, depends on howmany bits there are per symbol.

Asynchronous and Synchronous Data

The transmission of data occurs in either an asynchronous or synchronousmode.

In asynchronous data transmission, each character has a start and stop bit(Figure 5-1). The start bit prepares the data receiver to accept the character.The stop bit brings the data receiver back to an idle state.

Synchronous data transmission eliminates the start and stop bits. This typeof system uses a preamble (a known sequence of bits, sent at the start of amessage, that the receiver uses to synchronize to its internal clock) to alertthe data receiver that a message is coming.

Asynchronous systems eliminate the need for complex synchronization circuits, but at the cost of higher overhead than synchronous systems. Thestop and start bits increase the length of a character by 25 percent, from 8 to 10 bits.

CHAPTER

5DATA COMMUNICATION VIA VHF/UHF RADIO

F rom the very beginning, radio communications used Morse codefor data communications. Over time, improved techniques were developed for data transmission that take into account the

variability of the radio medium and greatly increase the speed at whichdata transmission occurs over a radio link. In addition, the applicationof error-correcting codes and automatic repeat request (ARQ) techniques offering error-free data transfer permits the use of radio transmissionsfor computer-to-computer communications systems.

To understand the principles of radio data communication, we’ll definesome common data terminology and explain the significance of the modem. We will also outline some of the problems and solutions associated with radio data communication.

Binary Data

Communication as an activity involves the transfer of information from a transmitter to a receiver over a suitable channel. Consider this book, for instance. It uses symbols (the alphabet) to encode information into a set of code groups (words) for transmission over a channel (the printed page)to a receiver (the reader). Applying this principle to data (information), we begin by using a kind of shorthand to transform the data into codewords (binary digits or bits) for transmission over a channel (HF radio) to a receiver (the reader).

Bits are part of a number system having a base of two that uses only thesymbols 0 and 1. Thus, a bit is any variable that assumes two distinct

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Radio Modems

Radios cannot transmit data directly. Data digital voltage levels must beconverted to radio signals, using a device called a modulator, which appliesthe audio to the transmitter. Conversely, at the receiver, a demodulatorconverts audio back to digital voltage levels. The Harris radios are equippedwith built-in high-speed modems (the MOdulator and the DEModulatorpackaged together), which permit the radios to operate with either voiceor data inputs.

Radio modems fall into three basic categories: (1) modems with slow-speedfrequency shift keying (FSK); (2) high-speed parallel tone modems; and (3)high-speed serial (single) tone modems.

The simplest modems employ FSK to encode binary data (0s and 1s) (seeFigure 5-2). The input to the modulator is a digital signal that takes one of two possible voltage levels. The output of the modulator is an RF signalthat is one of two possible tones. FSK systems are limited to data rates less than 75 bps due to the effects of multipath propagation.

Amplitude Shift Keying (ASK) is similar to FSK except that it is the amplitude of the carrier that is modulated rather than the frequency.

Higher rates are possible with more modern Phase Shift Keying (PSK) modulation methods and advanced coding schemes. PSK is described later in this chapter.

Error Control

There are several different approaches to avoid data transmission problems.

Forward Error Correction (FEC) adds redundant data to the data stream to allow the data receiver to detect and correct errors. An important aspect of this concept is that it does not require a return channel for the acknowledgment. If a data receiver detects an error, it simply corrects it and accurately reproduces the original data without notifying the datasender that there was a problem. Downsides of FEC: Unlike ARQ, FEC does not ensure error-free data transmission; FEC decreases the effectivedata throughput.

The FEC coding technique is most effective if errors occur randomly in adata stream. The radio medium, however, typically introduces errors that

57 58

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occur in bursts — that is, intervals with a high bit error ratio (BER) in thechannel are interspersed with intervals of a low BER. To take full advantageof the FEC coding technique, it’s best to randomize the errors that occur in the channel by a process called interleaving (Figure 5-3).

For example, at the modulator, the data stream enters a 9-row by 10-column matrix. The blocks are entered by rows and unloaded bycolumns. When the data stream leaves the matrix for transmission, the sequence of output bits will be 1, 11, 21, and so on.

At the demodulator, de-interleaving reverses the process. Data is enteredby columns in a matrix identical to that at the transmitter. It is read out inrows, restoring the sequence of data to its original state. Thus, if a burstwere to cause 9 consecutive bits to be in error, no more than 3 of them will fall in any 30-bit sequence of bits after de-interleaving. Then, if an FEC coding technique were used, the errors would be corrected.

Soft-decision decoding further enhances the power of the error-correctioncoding. In this process, a group of detected symbols that retain their analog character are compared against the set of possible transmitted code words. The system “remembers” the voltage from the detector and applies a weighing factor to each symbol in the code word beforemaking a decision about which code word was transmitted.

Vocoder

Data communications techniques are also used for encrypting voice calls by a device called a vocoder (short for voice coder- decoder). The vocoderconverts sound into a data stream for transmission over an HF radio channel. A vocoder at the receiving end reconstructs the data into telephone-quality sound.

Channel Equalization and Excision Filtering

In addition to error correction techniques, high-speed serial modems mayinclude two signal-processing schemes that improve data transmissions. An automatic channel equalizer compensates for variations in the channelcharacteristics as data is being received. An adaptive excision filter seeksout and suppresses narrowband interference in the demodulator input, reducing the effects of co-channel interference, that is, interference on thesame channel that is being used. Harris has patented several techniques to perform these functions.

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Modern High Data Rate Modem Waveforms

High-speed modem technology, permits data rates as high as 64 kbps.Radio transmission paths have varying characteristics depending upon thefrequency band (HF, VHF, and UHF) and the bandwidth of the channel.Although most HF channels are bandwidth limited to 3 kHz; VHF, UHF, andSATCOM channels have both 5 kHz and 25 kHz bandwidths. To accommo-date and maximize the data throughput rate for these radio transmissiontypes, a number of robust data waveforms have been created. Table 6-1lists these different waveform types and their applications.

TABLE 6-1

Waveform Channel Data Rate Application Bandwidth in kbps

ASK 25 kHz 16 kbps UHF HAVEQUICK

FSK 25 kHz 16 kbps VHF SINCGARS

PSK 5 and 25 kHz 2.4 kbps SATCOM DAMA

4-ary CPFSK 5 kHz 4.8 k to 9.6 kbpsSATCOM DAMA

25 kHz 9.6 k to 56 kbpsSATCOM DAMA

16-ary TCM 25 kHz 64 kbpsVHF/UHF

M-ary CPM 5 kHz 4.3 k to 8.5 kbpsVHF/UHF

25 kHz 21 k to 64 kbpsVHF/UH

Phase Shift Keying (PSK)

PSK is similar to FSK, shown in Figure 5-2, except that it is the phase of thecarrier rather than the frequency that is modulated.

61 62

Binary Phase Shift Keying (BPSK)

The simplest form of PSK is called Binary Phase Shift Keying (BPSK) shownin Figure 5-4. Figure 5-4a shows a reference wave covering two bit periods.Figure 5-4b shows the wave after modulation with a (0) bit and a (1) bit.Notice that the signal corresponding to the second bit (1) is an upside-down version of the reference waveform. This portion of the signal is 180˚ with respect to the reference waveform.

Notice also that the transition from the first bit to the second is abrupt. Thissudden phase discontinuity creates a burst of noise sidebands referred to as“splatter.” This noise causes inter-symbol interference which severely limitsthe data rate that this simple form of PSK can deliver.

M-ary PSK

There are many forms of PSK. BPSK is modulated with just two phases ofthe carrier. Another term for BPSK is 2-ary PSK. In this case M=2. Figure 5-5 shows a diagram that represents M-ary PSK by showing vectors that represent the phase angles associated with the most common types of M-ary PSK modulation.

BPSK is represented by two arrows facing away from each other at a 180˚ angle. Each of the two phases of BPSK can represent only one bit of information, either a (0) or a (1).

Quadrature Phase Shift Keying (QPSK), or 4-ary PSK, is shown with four arrows arranged around a circle so that each is 45˚ apart. Since there arefour phase states used in this modulation, each of these phases can represent two bits of information. Going clockwise around the circle, these bits are (00), (01), (10), and (11). This multi-bit representation perphase is the key to faster data rates, because each phase represents two bits rather than just one.

The figure also shows 8-ary PSK modulation, in which each phase represents three bits. Finally, 16-ary PSK is shown. Each phase representsfour bits of information. On a non-noisy radio channel, 16-ary PSK has a data rate that is four times faster than BPSK because each modulationphase state represents four times as many bits.

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63 64

Continuous Phase QPSK

Figure 5-6a shows what the waveforms of QPSK look like for each of thefour possible modulation states of (00), (10), (10), and (11). Each of thesebit pairs represents a code symbol.

Figure 5-6b shows a QPSK waveform covering two symbol periods in whichthe symbols change from (00) to (10). Notice that although this requires an 180˚ shift, there is no sudden discontinuity in the waveform. This is because a transition period equal to half of the symbol period has beentaken to gradually change the phase. Although this slows down the datarate, the extra time is made up by the decrease in discontinuity noise (splatter) and attendant inter symbol interference.

Noise Margin

The problem with PSK waveforms with M = to 8 or 16 is that the differencein phase between each modulation state is very small. For example, in 8-aryand 16-ary PSK, the phase difference between the (0000) and (0001) symbols is only 45˚ and 22.5˚, respectively. The noise margin is only half of those values because any noise that would make the signal appear to behalf way between the true values would yield a doubtful decision. Thus thenoise margin for 8-ary and 16-ary PSK is only 22.5˚ and 12.5˚, respectively.

In a noisy radio channel, such a narrow phase difference is much harder todetect than the 90˚ noise margin of the two possible phase states in BPSK forthe symbols (0) and (1). So, although 16-ary PSK can be four times as fast asBPSK in a perfect channel, it may be totally unreadable in a noisy channel.

The phase difference between adjacent phase states in a PSK scheme iscalled its “noise margin”. The greater this noise margin, the more immuneto noise this symbol transition is.

BPSK may be slow, but it is very robust in a noisy channel.

Trellis Coded Modulation (TCM)

Figure 5-7 (A0) is a representation of an 8-ary PSK phase diagram where thelinear distance between the arrows of adjacent phase points is labeled (d).As mentioned above, the noise margin corresponding to this distance is22.5˚. The term “distance” is another way of referring to noise margin.

180˚(10)

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Noise Margin = ± 90˚

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Noise Margin = ±45˚

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67 68

The distance between successive symbols in a data stream can be maximized by partitioning into code subsets having increasing distance between their elements. Starting from 8-PSK constellation (in Figure 5-7A0), we can create two 4-PSK subsets by taking every other signal point on the circle and putting them in one set and the rest of the signal pointsinto another set (sets B0 and B1). The distance between adjacent phaseson each of these sets is 1.85 times (d).

Each of the resulting 4-PSK sets can be further partitioned into two BPSK subsets (C0, C1, and C2, C3). The distance between the two signalpoints in each BPSK subset is 2.6 times (d). Considering all combinations of phases for each constellation, there are a total of six subsets of the basic 8-PSK signal set.

Each choice of subset, including the choice of one of the BPSK symbols in the last set, is assigned a bit value for a total of three bits. Because each bit has a different signal distance associated with it, each bit has a different likelihood of error.

The bits with the highest likelihood of error are coded into subsets with agreater distance between bits. The effect of coding is to make the signaldifferent over multiple symbols due to the bit input at the present symbol.Distance is now measured over the several symbol intervals allowing thesignal to “build up” more distance for any bit decision.

This process of subset partitioning and coding is called Trellis CodedModulation. This basic concept can be extended to a 16-ary PSK signalwith a bit rate of up to 64 kbps in a 25 kHz bandwidth radio channel.

SUMMARY

■ The transmission of data requires the use of modems to convert digital data RF signal form when transmitting, and convert the RF signal back to digital form when receiving.

■ Radio modems are classified as slow-speed FSK, high-speed parallel tone, or high-speed serial tone.

■ Serial tone modems provide vastly improved data communications,including a higher data rate with powerful forward error correction (FEC), greater robustness, and reduced sensitivity to interference.

■ FEC systems provide error correction without the need for a return link.

■ Interleaving is a technique; mostly used for HF channels that randomizes error bursts, allowing FEC systems to work more effectively.

■ Soft-decision decoding further reduces bit error rates by comparing a group of symbols that retain their analog character against the set of possible transmitted code words.

■ A vocoder converts voice signals into digital data for codedtransmission over HF channels.

■ Automatic channel equalization and adaptive excision filtering aresignal processing techniques that improve data communicationsperformance.

■ M-ary Phase Shift Keying is a method of increasing the data rate ofradio transmissions. “M” refers to the number of phases used in the modulation scheme.

■ Trellis Coded Modulation (TCM) is a coding technique that provides maximum data rate capability to PSK data streams by improving the noise margin.

Technically, any ground radio with no obstructions above it is within theLOS of any satellite that is above the horizon. Chapter 2 stressed the advantage that antenna height makes in extending LOS distance. A satellite is the ultimate high antenna tower.

UHF is an excellent candidate frequency to contact a satellite because it can penetrate the atmosphere and ionosphere with little attenuation.

Uplink and Downlink Frequencies

The function of a repeater is to receive a radio signal at a particular frequency, amplify it, and then convert it to another frequency for rebroadcast. The radio paths up and back from a satellite are called uplinks and downlinks.

Different uplink and downlink frequencies are required to avoid feedbackbetween the satellite transmitter and receiver. UHF uplink frequenciesrange from 292.95 MHz to 310.95 MHz, while downlink frequency range from 250.45 MHz to 269.95 MHz.

SATCOM SatellitesFigure 6-1.

69 70

CHAPTER

6

T he increased requirement for greater troop mobility has been accommodated by the trend toward multiband, multimoderadio systems. While technology permits the production of smaller,

lighter-weight radio equipment, overall communications capability has notkept up with the demand. The need for flexibility, security, and reliabilityof terrestrial radio communications remains a critical problem.

The two most significant radio limitations are the congested frequencyspectrum and the physical limits on radio wave propagation. The develop-ment and use of communications satellites are an attempt to overcomethese limitations.

Even as there is a need for HF, VHF, and UHF radio in the tactical environment, there is also a need for different types of satellite systems.These can also be grouped by frequency bands as follows: Ultra HighFrequency (UHF), Super High Frequency (SHF), and Extra High Frequency(EHF). Figure 6-1 summarizes the chief characteristics of these threegroups.

Figure 6-1 shows that the outstanding characteristics of the UHF group are the mobility of the ground terminals and its overall lower cost. It is thisgroup that is used by our tactical mobile forces. The SATCOM discussionsin this volume will be limited to this UHF Satellite system.

A Communications Satellite is a Radio Repeater

Just as cell phones use radio repeaters placed on towers and tall buildingsto increase area coverage, SATCOM transceivers achieve coverage by using radio repeaters placed in satellites.

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Uplink and downlink frequencies are paired within specific channel groupsand frequency plans within that group. For example, Channel 2, Plan A,specifies an uplink frequency of 251.95 MHz and a downlink frequency of 292.95.

SATCOM transceivers can be programmed so that when in the SATCOMmode, a transceiver adjusted for a given channel will automatically choosethe correct uplink and downlink frequencies for transmitting and receiving.

The Geostationary Orbit, and Coverage Footprint

The laws of physics are such that the speed of a stable satellite orbit depends upon its distance above the earth. If a satellite is placed in a stable orbit 22,300 miles above the equator, it must travel just fast enoughto make a rotation around the earth in 24 hours. Since that is exactly thesame speed that the earth rotates, a satellite placed in that orbit will hoverover the same spot on earth as they both rotate together. This is called a geostationary orbit. Satellites closer to the earth must travel faster to remainin orbit and their positions would drift around the earth to the east.

A great advantage of having a communications satellite in a geostationaryorbit is that it has a fixed, huge LOS coverage area. Figure 6-2 representsoverlapping LOS areas created by having four such satellites evenly distributed around the earth above the equator. These LOS areas are called footprints of the satellite. Just four of these satellites provide foot-prints that cover the earth from the latitudes of 70˚ north to 70˚ south.

A ground transceiver located anywhere within a footprint can link with the associated satellite, and then back down to any other transceiverlocated within the footprint. For example, Figure 6-2 shows that a transceiver located anywhere in North America can link with a transceiver located anywhere in South America.

Many locations are under two adjacent footprints. This gives two possiblesatellite path choices. For example, a transceiver located near the EastCoast of the US is within the footprints of both satellites FLT1 and FLT3.The footprint of FLT3 includes all of Europe and Africa. Being able to use both of these footprints provides a range that includes most the US, South America, Europe, and Africa.

71 72

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73 74

With the use of ground repeating relay stations, the communication rangeof a SATCOM transceiver can have worldwide coverage.

SATCOM Antennas

The bad news about geostationary satellites is that they must be 23,300miles above the earth’s equator. That is a very long LOS distance for a relatively low power UHF transceiver. (Manpack SATCOM transceivers are generally limited to 20 watts or less of transmit signal power.)

The good news about geostationary satellites is that their exact locationis known, so the LOS direction to it from any place within its footprint can be calculated.

To make the most of both the good and bad news, the antennas used for SATCOM work are directional (Figure 6-3). That is, they are constructedwith a reflector similar to those used in a flashlight to focus the beam. By focusing the beam of a directional antenna, you can boost the effective radiated power by four times or more. Ground troops in a given theater of operation are told the precise compass bearing and elevation to aim the antenna so that it points directly toward the desired satellite.

To achieve a greater margin of link closure, vehicular and fixed station applications usually include adapters that provide amplification of the manpack transmit signal to 50 watts.

UHF SATCOM Channel Characteristics

As an example of UHF SATCOM channel characteristics, the U.S. Navy has a group of satellites called Fleet SATCOM (FLTSATCOM) frequency channels. The channel capabilities of a FLTSATCOM satellite are as follows:

■ One 25-kHz channel downlink with a special 15 kHz SHF uplink dedicated to Navy fleet broadcast use.

■ Nine 25-kHz relay channels for general use.■ Twelve Air Force narrowband, 5-kHz channels.■ One DOD wideband, 500-kHz channel for special use.

The fleet broadcast channel mentioned above is a one-way, shore-to-shipchannel of 25-kHz bandwidth, which supports 15 time-division multiplexed, 100 wpm Teletype circuits. Its uplink is transmitted as an

75 76

SUMMARY

■ There are UHF, SHF, and EHF military satellites.

■ UHF satellites are used for tactical military use, which includes ground forces as well as those of the Navy and Air Force.

■ A footprint of a satellite is the total ground area for which a LOS path exists.

■ A UHF SATCOM transceiver can link with any satellite that includes the transceiver in its footprint.

■ The use of multiple footprints and ground relay stations can extend the SATCOM range to nearly the entire world.

■ The Satellite Management Center (SMC) regulates and assigns the satellite resources to users.

■ Directional antennas must be used with UHF SATCOM transceivers.

■ Demand Assigned Multiple Access (DAMA) is a way to timeshareavailable satellite resources in an efficient way.

anti-jam protected SFH signal, which is then processed, and frequencytranslated to a UHF downlink by circuits within the satellite.

The nine 25-kHz channels for general use are dedicated to FM modulatedsignals. Any data waveform that results in an FM, 25-kHz bandwidth cantake advantage of these channels.

Demand Assigned Multiple Access (DAMA)

The few channels available from each satellite require that strict controlsmust be enforced about sharing. Each theater of operation has a SatelliteManagement Center (SMC) that is located away from the immediate battle zone, but is within communication distance of those within the battle zone. An operation that requires the use of SATCOM must get aplan approved from the SMC. This plan includes specific designated channels and channel access protocols.

One of the widely used protocols is Demand Assigned Multiple Access(DAMA). This is a technique that matches user demands to available satellite time.

Satellite channels are grouped together as a bulk asset, and DAMA assignsusers variable time slots that match the user’s information transmission requirements. The user notices no difference — it seems he has exclusiveuse of the channel. The increase in nets or users available by using DAMAdepends on the type of users. DAMA is most effective where there aremany users operating at low to moderate duty cycles. This describes many tactical nets; therefore, DAMA is particularly effective with TACSAT systems.

DAMA efficiency also depends on how the system is formatted. Formattinga DAMA system is how the access is controlled. The greatest user increaseis obtained through unlimited access. This format sets up channel use on a“first-come-first-serve” basis. Other types of formats are prioritized cuingaccess and minimum percentage access.

The prioritization technique is suitable for command type nets, while the minimum percentage is suitable for support/logistic nets. Regardless of format, DAMA generally increases satellite capability by 4 times over normal dedicated channel operation.

77 78

CHAPTER

7

I nformation security is becoming a high priority for businesses aroundthe world. With the dramatic increase in electronic communicationsand electronic commerce, there has been a corresponding increase in

the malicious compromise of that information. In this chapter, we’ll discusscommunications security (COMSEC), that is; methods that keep importantcommunications secure. We’ll also talk about transmission security(TRANSEC) — schemes that make it difficult for someone to interceptor interfere with your communications.

COMSEC

COMSEC uses scrambling or cryptographic techniques to make informa-tion unintelligible to people who do not have a need to know or whoshould not know. We’ll differentiate here between cryptographic or ciphering techniques applied to digital signals and scrambling techniquesapplied to analog signals.

Cryptography is the process of encrypting (translating) information into an apparently random message at the transmitter and then decipheringthe random message by decryption at the receiver.

Historically, sensitive information has been protected through the use of codes. The sender would manually encode the messages before transmission and the recipient would manually decode the messages upon receipt. Today’s electronic technologies allow the coding/decodingprocess to occur automatically.

The process involves using a mathematical algorithm, coupled with a key,to translate information from the clear to the encrypted state. If sensitiveinformation is transmitted without the protection of cryptography and the information is intercepted, it would require little effort or resources

SECURING COMMUNICATIONS

to understand the transmittal. The US Government has established stan-dards for the degree of protection required for different levels of classifiedand sensitive information.

In voice communications systems that do not require extremely high security, you can protect against casual eavesdropping by scrambling.Scrambling, as an analog COMSEC technique, involves separating the voice signal into a number of audio sub-bands, shifting each sub-band to a different audio frequency range, and combining the resulting sub-bandsinto a composite audio output that modulates the transmitter. A randompattern controls the frequency shifting. The technique of scrambling thepattern is similar to sending a message with a decoder ring, like the onessometimes found in children’s cereal boxes. You can, for example, designate that the letter c be ciphered as g, a as n, and t as w, so thatwhen you receive the message gnw, you decode it as cat. Descramblingoccurs at the receiver by reversing the process. In today’s digital age, analog scrambling has given way to digital encryption.

Digital Encryption

To digitally encrypt a transmission, analog voice information must be firstdigitized by a VOCODER (as mentioned in Chapter 5), which converts thesignal into a binary data stream.

The binary data stream is then applied to what is called a “cryptographicengine.” This is a processor which creates an extremely long, non-repeating binary number stream based on a complex mathematical algorithm and a traffic encryption key (TEK). The TEK is a binary numberthat is used to control the algorithm.

Binary addition is then used on a bit by bit basis to merge the crypto-graphic stream with the data stream. A binary stream created in this fashion is inherently unpredictable, and bears little resemblance to the original data stream. It is now called encrypted data or cipher text.Decryption can only be accomplished by knowing the algorithm and the TEK, and then by reversing the encryption process.

The data encryption strength is a function of the complexity of the mathematical algorithm coupled with the TEK (sometimes just called the key). Protection of the key is vital.

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Even if an unwanted organization gains access to the encrypted informa-tion and has the algorithm, it is still impossible to decrypt the informationwithout the key. The US Government has developed rigorous key manage-ment procedures to protect, distribute, store, and dispose of keys.

In the past, keys were manually loaded into a cryptographic device by usinga paper tape, magnetic medium, or plug-in transfer device. Creation andsecure delivery of keys to each user were significant problems in both logistics and record keeping.

One type of key management system also used in the commercial sector is public key cryptography. Under this standard, each user generates twokeys. One is the public key, “Y,” and the other is the private key, “X.” TheY value derives from the X value. The strength of such a system lies in thedifficulty of deriving X from Y; what is encrypted with the Y key can only be decrypted with the X key. By openly disseminating the user’s public Ykey, and retaining sole access to the private X key, anyone can send a secure message to you by encrypting it with your public Y key. You are the only one, though, who can decrypt the message, since only you have the private X key.

In a network using this public key system, two-way secure communicationsare possible among all network users. This is called an asymmetrical keysystem. The alternative is a symmetric key system, in which the same keyencrypts and decrypts data. Because both the originator and all recipientsmust have the same keys, this system offers the highest levels of security.Harris has led the way in developing state-of-the-art electronic means tosecure and distribute key material for these symmetric key-based communications systems.

A recent development applicable to radio networks employs Over-The-Air-Rekeying (OTAR). This technique nearly eliminates the need for manualloading of keys and provides a secure key management.

OTAR is based upon a benign key distribution system. It includes a key encryption key (KEK) used to encrypt the TEK and any other operationalCOMSEC or TRANSEC keys. This process is referred to as “wrapping” todifferentiate it from traffic encryption. The KEK is the only key that must be initially loaded into both the sending and receiving units. Usually, an initial set of operational keys is loaded at the same time.

After wrapping, subsequent distribution can use any physical or electronicmeans. In an OTAR system, the wrapped keys are inserted into a messageand sent over a radio link to the intended station using error-free transmission protocols (an error would render the keys useless). The linkused for transmission is usually secured by the TEK currently in use. Thus,the key material is doubly protected when sent over the air, practically eliminating any possibility of compromise.

TRANSEC

TRANSEC employs a number of techniques to prevent signal detection or jamming of the transmission path. These techniques include hiding the radio transmission or making it a moving target.

Low Probability of Detection (LPD) systems hide the radio transmission by transmitting it using very low power, or by spreading the signal over a broad bandwidth so that the natural noise in the environment masks the signal.

The most commonly used TRANSEC technique is frequency hopping. In this system, the transmitter frequency changes in accordance with a com-plex algorithm so rapidly that it is difficult for an unauthorized person to listen in or to jam the signal. The receiver is synchronized so that it hopsfrom frequency to frequency in unison with the transmitter. A TRANSECkey system modifies the hopping algorithm so that only transmitters and receivers that use the same key can communicate.

Frequency hopping scatters the intelligence over several hundred discretefrequencies. A radio operator listening to one of these frequencies mayhear a short “pop” of static. A broadband receiver could perhaps captureall of these little bursts; however, the task of picking these bursts out of theother natural and man-made bits of noise would be daunting, requiring ateam of experts several hours just to reassemble a short conversation.

Jamming one channel would have minimal impact on the hopping communicator. To effectively jam a frequency-hopping radio, most or all of the frequencies that the hopping communicator uses would have to be jammed, thus preventing the use of those frequencies as well. HarrisCorporation’s AN/PRC-117, AN/PRC-138, FALCON and FALCON II transceivers are highly rated for their frequency-hopping capabilities.

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National Security Agency (NSA) Certification

The inclusion of COMSEC and TRANSEC capabilities into radio equipmentrequires stringent design practices to ensure that not even a trace amountof the unencrypted signal gets inadvertently transmitted along with the encrypted signal.

For example, an analog voice signal applied to the input of a radio has atendency to cause slight fluctuations in the radio power supply that can actually amplitude modulate the output power amplifier of the radio. If this happens, a sensitive receiver can detect the unencrypted audio signal.

Having a copy of both an original and encrypted message not only givesthe enemy the specific unencrypted message, but places in jeopardy anysignals transmitted with that same TEK and algorithm.

Similarly, the cryptographic stream created by the COMSEC engine can“leak” to the output through the power supply or because of inadequateinternal shielding. If the enemy has a copy of the cryptographic stream, itcan be used to decode the encrypted data.

To avoid these and other similar problems, an impenetrable interface mustbe designed into the radio and the COMSEC and TRANSEC modules thatkeep the unencrypted signals totally separated from the circuits that createthe radio frequency signal. Those circuits that are associated with unen-crypted input signal are called “Red.” Those associated with the encryptedsignal are called “Black.” Red/Black interface is the barrier between them.

In order for a manufacturer to furnish COMSEC and TRANSEC modulesand radios for high-grade US Government use, a thorough testing programmust be designed and then approved by the National Security Agency. The radios are then meticulously tested by NSA experts to ensure that not a trace of unencrypted signals escape into the radio frequency signalstream. Only after passing many such tests can a company be certified to produce this high-grade type of cryptographic equipment.

Harris Corporation, RF Communications Division, is a supplier of NSA-certified products and is a preferred supplier of information security for the US Government and the US Department of Defense. It is a leader inthe development and production of US Government and exportable security products. The company also provides a comprehensive line of secure products for the non-US Government market.

Harris radios have a wide variety of modern COMSEC and TRANSEC engineoptions. These engines are also available as modules for incorporation inOEM hardware.

Presidio

Presidio is a high-speed full or half-duplex embeddable US governmentCOMSEC module, used to secure digital voice or data traffic over radio,wireline or other telecommunications media. Presidio is capable of data encryption/decryption at speeds up to E1 (2.048 Mbps) data rate. Presidiooffers COMSEC equipment manufacturers a wide range of interoperabilityand key management features as well as reduced size, weight and numberof devices required, making Type 1 certification an easier process.

CITADEL™

The CITADEL cryptographic engine provides high-grade protection for USdomestic and international customers over all modern communicationsmedia. It is available with configurable key lengths and multiple algorithmoptions, making CITADEL an ideal export encryption solution for a broadrange of communication products. The CITADEL supports both COMSECand TRANSEC functions allowing the device to be adapted to virtually anycommunication environment.

Sierra™

The Sierra module addresses the need for an encryption technology thatcombines the advantages of the government’s high-grade security with the cost efficiency of a reprogrammable, commercially produced encryptionmodule. It provides a common security solution to users that can take onmultiple encryption personalities depending on the mission that has been programmed.

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SUMMARY

■ COMSEC uses cryptography or scrambling to make informationunintelligible to people who do not have a need to know or who should not know.

■ The security level of a COMSEC system depends on the mathematical complexity of the algorithms and the number of variables in the key.

■ Protection of the key is vital to securing the transmitted information.

■ Public key cryptography is widely used in the commercial sector.

■ Over-The-Air-Rekeying (OTAR) eliminates the need for manual loading of keys and provides a more secure method of key management.

■ TRANSEC protects the transmitted signal itself, to prevent signaldetection or jamming of the transmission path.

■ Low Probability of Detection (LPD) systems use spread-spectrum and other techniques to “hide” the signal beneath the natural noise level.

■ Frequency-hopping radio systems jump rapidly in unison, from one frequency to another in apparently random patterns, using a common timing reference.

■ Presidio, CITADEL, and Sierraare modern COMSEC and TRANSECengines.

CHAPTER

8

T oday’s communications system designer makes use of HF, UHF,VHF, and SATCOM tools and capitalizes on the unique capabilitiesthat each brings to the job. HF radio offers a unique combination

of cost effectiveness and versatility for long-haul communications, whileVHF/UHF radio products bring solutions to classical line-of-sight communi-cations requirements.

Recent developments in digital signal processing and manufacturing miniaturization have pushed the technology to smaller, lighter, and less expensive equipment. Software-based radio platforms are now able tosolve complex system requirements. This chapter can only touch on a few of the many modern radio system possibilities available today.

The Digital Battlefield

Figure 8-1 illustrates a modern battlefield communications network architecture that uses SATCOM, UHF, VHF, and HF technologies.

Mobile communications vehicles provide hubs that receive and retransmitradio signals that cover the entire available spectrum. VHF Combat NetRadios (CNR) provide ground LOS communications between squads, while UHF radios provide the same for ground-to-air (G/A) close support.

Harris VHF/UHF radios are interoperable with all common waveforms usedin aircraft. The operator can easily program channels on frequencies for aG/A net, in plain text AM or FM, as well as cipher text AM or FM.

HF and SATCOM radios provide long haul communications from the frontline back to theater headquarters. Such a tactical communications networkprovides coverage over distances ranging from less than 50 miles to morethan 1,500 miles.

SYSTEMS AND APPLICATIONS

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Individual elements of this network include frequency hopping, encryption,and HF Automatic Link Establishment (ALE) capabilities.

Network requirements dictate that links are provided between the fixedheadquarters site and fixed installations for quasi-permanent military regions and zones. Provisions are then made for communications betweenheadquarters and task forces at fixed, non-permanent, installations.

Lower echelon communications have a combination of fixed, mobile, and man-portable equipment. Frequency management of the network is a headquarters responsibility.

Video Imaging

VHF/UHF bandwidth considerations support 16 kbps to 64 kbps within a typical 25 kHz channel, improving the speed in which a text string, file, or document can be sent.

Figure 8-2 shows a scenario in which an unattended still-video camerasends images to an imaging terminal via a fiber-optic link. The terminalcaptures and digitizes the image and sends the data to a modem in thetransceiver, which relays the data to the base. Communications may be via a two-way link that uses an ARQ protocol to obtain error-free transmis-sion of the image, or a one-way link in which FEC coding reduces the probability of error in the received message.

High data rates for detailed images or maps shorten the on-air time required to pass the image files.

The Harris Universal Image Transmission System (HUITS) digital imagingproduct optimizes the compression of digital images and uses an ARQ protocol specifically designed for transmission over tactical radio channels.

VHF/UHF Telephone System

A VHF or UHF radio link extends the reach of a telephone network to thebattlefield as shown in Figure 8-3. The telephone system enables users toplace calls to and from mobile radio transceivers in the field to commercialswitched telephone networks or private subscriber telephone lines.

Calls from the field are placed over HF, VHF, or UHF to anywhere in theworld through the base station telephone switch or exchange. To initiateFi

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a call, the user enters a telephone number just as if the Remote Access Unit(RAU) was a telephone set connected directly to the base station telephoneexchange.

At this point, the number dialed is transmitted through the RAU to theTelephone Interface Unit (TIU). As the TIU dials the digits and the targettelephone rings, the mobile operator hears call progress tones, just as if a regular telephone was used.

In order to contact anyone in the field, a telephone user dials a telephonenumber (or the extension) to which the TIU is connected — from anywherein the world. The TIU automatically answers the call and the user is connected directly with the field radio.

Secure communications can be achieved through interfaces to the com-mon inventory of encryption devices. The high data rates available over a25 kHz VHF channel improve the intelligibility of wide-band secure voice.

Radio E-mail

Electronic mail and other inter-networking technologies are becoming increasingly important for interoffice communications. However, manyusers find that communications between remote stations are difficultand/or expensive, due to costly telephone or satellite charges. Harris hasdeveloped radios and systems that are an excellent alternative for providingthese services to distant users or stations. Typical applications include:

■ Naval ship-to-shore and ship-to-ship communications■ Embassy Ministry of Foreign Affairs communications■ Oil/Gas/Mining operations

In the following discussion, we will focus on naval applications; similar configurations support other radio e-mail and inter-networking communications system requirements.

A radio e-mail system for naval ships and deployed forces that supportsnaval communications, including administrative, logistic, and engineeringorder-wire traffic, is shown in Figure 8-4.

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connect to a Harris Wireless Gateway, which extends the intranet usingUHF LOS radio links to the various ships of the group. The Gateway provides seamless data transfers between common networked applica-tions, such as E-mail and FTP file transfer, running on geographically separated LANs. HF long haul BLOS radio links, can extend the group intranet to distant support groups or to headquarters on shore.

Applications such as the Harris Wireless Messaging Terminal make the operator interface nearly identical to the office environment e-mail handling applications. This makes training of personnel easier and message handling simpler.

System Design Considerations

Harris RF Communications Division has a communications systems engineering department staffed by specialists in the design of customequipment for the “one-of-a-kind” type of application. The following are some of the factors that systems engineers consider when designing a modern radio system.

System definition

■ Who are the users?■ What is their location? ■ Are communications one-way or two-way? ■ What are the interfaces with other communications media?■ What is the operating environment (hostile or friendly, rural or urban)?

Transfer of information

■ What type of traffic is there (voice, data, images)?■ Do the priority levels differ, depending on the message source and/or

content?■ What are the security levels for safeguarding the information?

Message protection and security

■ What is the correct type of error detection and correction for data?■ What type of COMSEC is needed?■ Will spread-spectrum or frequency-hopping techniques be used to

avoid interception or jamming?■ Is excision filtering needed to remove interfering signals?

System availability

■ What is the probability of transferring messages in real time?■ Can alternate routing be used to enhance message availability? ■ Can lower priority traffic use store-and-forward techniques?■ Are there any operational restrictions due to propagation,

transmitter power, or other constraints?

Traffic analysis

■ What are the typical message lengths?■ What is the average number of messages per unit of time?■ What are the message priorities?■ When is the peak traffic?■ What are the types of traffic?

Projected growth for each category of traffic

■ What impact does higher traffic levels have on system implementation? ■ Are additional nodes and/or relays necessary?

Impact on message structure

■ Is the format for data message compatible with traffic requirements? ■ Include security classification, priority, source, and destination address.

Fixed site

■ Are the receiving, transmitting, and control functions collocated or separate?

■ Is this a permanent or temporary installation?■ Are there any frequency restraints for collocated receivers

and transmitters? ■ What are the staffing requirements?■ What are the environmental considerations?■ What type of power is available?■ Is uninterrupted power a requirement?

Mobile site

■ Is the equipment designed for a vehicle, ship, shelter, or aircraft?■ Are manpacks required? ■ What are the antenna limitations and constraints? ■ What are the physical size constraints?

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SUMMARY

■ Modern radios are small and lightweight. Features and capabilities, which formerly required additional equipment, are now fully embedded into the radio transceiver.

■ HF, VHF/UHF and SATCOM radios play key roles in modern worldwide telecommunications systems, often working in conjunction with other media, such as cellular networks, and telephone landlines.

■ Radio e-mail is becoming a very important part of military communications systems.

■ A systems approach is needed to obtain the best results in designing a modern radio communications network.

■ What are the bandwidth and primary power requirements?■ What are the environmental considerations?

Communications protocol

■ Is there a return channel for ARQ? ■ Is ALE being used? ■ What are the data protocols?

Equipment selection

■ Transmitter requirements: Power output, solid state versus tube, broadband or narrowband, allowable distortion, frequency range, tuning speed, remote control?

■ Receiver requirements: Selectivity, dynamic range, distortion, remote control?

■ Antenna requirements: Gain, bandwidth, polarization, radiation pattern, available terrain, remote control?

Data communications systems

■ What is the data rate? ■ How is data being protected (type of encryption)? ■ What is the modulation scheme?

Interface to other equipment and systems

■ What other equipment is required (fax, data terminal, imaging systems)?

■ What other types of systems are involved? ■ What interfaces are required between HF/ VHF/UHF radio systems,

satellite, or switched telephone networks?

Command and control

■ Will operation be attended or unattended? ■ Is self-test required? ■ Are the transmitter, receiver, and control sites at different

places (split site)?

sizes are reduced through cuts in military spending, accurate situationalawareness is a key to maintaining the dominant level of force lethality. In the future, SA will become a background task of the command and controlcommunications structure. Position information will be securely appendedto all voice and data traffic, and routed to a SA collection point so that military commanders can plan and execute a successful military campaign.

Finally, increased sophistication of enemy forces will demand the use of improved information security (INFOSEC) techniques. This includes stronger,embedded encryption/decryption devices used for all voice and data traffic,advanced electronic counter-counter measures (ECCM) methods to sustaincommunications in the presence of intentional jamming, and the use ofLow-Probability of Intercept (LPI) and Low Probability of Detection (LPD)methods to protect forward-deployed troops operating in hostile areas.

Tactical Networking – Simple, Seamless and Secure Communications

During this past decade, we have all witnessed the impact the growth of the commercial Internet has had on countries around the globe. Itdemonstrates the power of seamless connectivity and the benefits gained from establishing common interfaces and protocols. Today the commercial Internet is starting to embrace the challenges presented by a wireless world. Many of these challenges are the same as those en-countered in a modern military communications system – the demand for seamless connectivity, self-forming and healing networks, and secure communication links – to name but a few. Successful military communica-tions equipment of the future will embrace this technology, building on the technological base established from enormous investments in the commercial sector.

Tactical networking will become an enabler for many military applications in the 21st Century. Example applications include: command/control sys-tems; situational awareness systems; automatic range extension; tacticalmessaging systems; fire-control systems; full duplex and simultaneousvoice/data systems; common database access; even combat net radio interface (CNRI) systems which link tactical and land-based infrastructures.

Radios, such as the Harris Falcon™ II Tactical Radio Family, will provide a seamless IP networking interface to other systems and application programs. New systems will be quickly and cost-effectively developed usingcommercial-off-the-shelf (COTS) tools and applications. Advanced channel

CHAPTER

9FUTURE DIRECTIONSby George HelmProduct Director, Tactical Business Unit

The first radios were substitutes for a pair of copper wires and were com-monly referred to as wireless communications. These radios were used tobridge gaps that couldn't be managed with wire lines, such as betweenships at sea and the shore. Later, as FSK, facsimile, video, and encryptionbecame popular, special purpose boxes were invented to encode and decode base band signals that were transmitted over these simple radio channels.

As the mobility of communications equipment became more and more important, the hardware in these external boxes were miniaturized and incorporated into the radios. The advent of powerful Digital SignalProcessing (DSP) chips and controlling microprocessors enabled the verycomplex coding and modulation schemes described earlier in this hand-book. These schemes have been used to increase the efficient use of bandwidth up to the point that is now approaching theoretical limits.

What challenges await military tactical radio communications equipment in the 21st Century? The answer to this can be found by looking at thechanges that have emerged on the modern battlefield. During the Gulf War, we witnessed a dramatic increase in the operational tempo of thebattlefield. Command and control communications were lost as the military forces moved rapidly across the flat desert terrain, outpacing theconventional military communication systems. Increased information flow demands the use of data communications in place of voice communi-cations, with an ever-increasing thirst for bandwidth. Critical battlefieldinformation must flow both horizontally and vertically, driving the need for tactical networking.

There also emerges an increased need for situational awareness (SA) –knowing the precise location of military assets and personnel. As force

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access protocols are under development to ensure the maximum effectiveuse of the available frequency spectrum. Packet data transmissions will enable multiple applications to be simultaneously supported on a priority-driven basis and will allow voice and data traffic to coexist, even on a single, narrowband radio channel.

Packet data transmission, combined with very fast receive-to-transmitswitching techniques, allow a simplex channel (one that alternately trans-mits and receives on the same frequency) to emulate the performance offull-duplex channels (one that receives and transmits on different frequen-cies simultaneously). It used to take two radios for retransmission, one toreceive and the other to transmit simultaneously on another frequency.Cutting edge radios of today can accomplish this with only one radio.

The trend in this area is to build a seamless network supporting any combi-nation of point-to-point and point-to-multipoint voice and/or data connec-tions. Information will be encrypted and decrypted only at the originationand destination stations, ensuring information securing from end-to-end.

Higher Speed Data

During conflict, timely information collection and dissemination is critical to operating within the enemy’s decision cycle. This need for increasedinformation flow is driving the desire for ever-increasing data transmissionrates. The latest generation of tactical radios offers a 4x increase over thosecurrently fielded. This increase in data rate significantly increases messagethroughput and reduces the message latency.

According to Chester Massari, Harris RF Communications division presi-dent, "Data transmission is the future of military communications. As one of the pioneers in this area, we can send high-speed imagery anddata across the radio spectrum better than anyone in the industry – plus, we offer this capability for land, sea, and air missions."

Where Do We Go From Here?

It is natural to ask the question, where do we go from here? Of course,with technology expanding at an ever-increasing rate, it is difficult to predict specifics more than a year or so in the future, but these are some of the important trends that are likely to influence radio technology for many years to come.

As we said in the introduction to this handbook, "Stay tuned!"

GLOSSARY

ADAPTIVE EXCISION FILTER — A signal-processing technique thatimproves data transmissions. It seeks and suppresses narrowband interfer-ence in the demodulator input and reduces the effects of co-channelinterference (interference on the same channel that is being used).

ALE (Automatic Link Establishment) — A technique that permits radio stations to make contact with one another automatically.

AM (Amplitude Modulation) — A technique used to transmit informationin which the amplitude of the radio frequency carrier is modulated by theaudio input and the full carrier and both sidebands are transmitted.

AME (Amplitude Modulation Equivalent) — A method of single sidebandtransmission where the carrier is reinserted to permit reception by conventional AM receivers.

AMPLITUDE — The peak-to-peak magnitude of a radio wave.

ANTENNA COUPLER/TUNER — A device between the transmitter and antenna that modifies the characteristics of the load presented to the transmitter so that it transfers maximum power to the antenna.

ARQ (Automatic Repeat Request) — Data transmission technique for error-free data transfer.

ASK (Amplitude Shift Keying) — A form of modulation in which a digital signal shifts the amplitude of the carrier.

ASYMMETRICAL KEY SYSTEM — A key management system thatallows two-way secure communications among all users that have a public key and a private key.

CARRIER — A radio frequency signal that may be modulated with information signals.

CHANNEL — A unidirectional or bi-directional path for transmittingand/or receiving radio signals.

CIPHER TEXT — Encrypted data.

CNR — Combat Net Radios.

COLLOCATION — The act or result of placing or arranging side by side.

COMSEC (Communications Security) — Scrambling or cryptographic techniques that make information unintelligible to unauthorized persons.

COSMIC NOISE — Random noise originating outside the earth’satmosphere.

CRYPTOGRAPHY — A COMSEC technique that translates (encrypts)information into an apparently random message and then interprets(deciphers) the random message by decryption.

CW (Continuous Wave) — A radio wave of constant amplitude and constant frequency. Also, Morse code.

DAMA (Demand Assigned Multiple Access) – A technique that matchesuser demands to available satellite time.

dB (Decibel) — The standard unit for expressing transmission gain or loss and relative power ratios.

DE-INTERLEAVING — Process used by a demodulator to reverseinterleaving and thus correct data transmission errors used in FEC coding.

DEMODULATION — The process in which the original modulating signal is recovered from a modulated carrier.

DIFFRACTION — When a VHF or UHF wave comes to a sharp edge, a portion of the wave bends around the edge and continues propagation as if a very low power radio was placed at the top of the ridge.

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ASYNCHRONOUS — A data communication system that adds start-and-stop signal elements to the data for the purpose of synchronizing individualdata characters or blocks.

ATMOSPHERIC NOISE — Radio noise caused by natural atmosphericprocesses (primarily by lightning discharges in thunderstorms).

ATTENUATES — Decreases.

AUTOMATIC CHANNEL EQUALIZER — A signal processing techniquethat improves data transmissions by compensating for variations in thechannel characteristics as data is received.

BANDPASS FILTER — A filter that passes a limited band of frequencies. It removes noise and spurious signals generated in the exciter or output frequency harmonics from the power amplifier.

BANDWIDTH — The range of frequencies occupied by a given signal.

BAUD — A unit of signaling speed equal to the number of symbols, i.e., discrete signal conditions per second.

BER (Bit Error Ratio) — The number of erroneous bits divided by the totalnumber of bits communicated.

BICONICAL ANTENNA — An antenna used for fixed station use; designed to cover the 100 to 400 MHz range.

BINARY — Number system having base of 2, using only the symbols0 and 1.

BIT — One binary digit (0 or 1).

BLOS (Beyond Line-of-Sight) — Communications that occur over a greatdistance.

BROADBAND — A term indicating the relative spectrum occupancy of a signal as distinguished from a narrowband signal. A broadband signaltypically has a bandwidth in excess of twice the highest modulatingfrequency. Synonym: Wideband.

FLTSATCOM (Fleet SATCOM) — A group of Navy satellites.

FM (Frequency Modulation) — A form of modulation where the frequencyof a carrier varies in proportion to an audio modulating signal.

FOOTPRINT — The line of sight (LOS) areas covered by a satellite.

FREQUENCY — The number of completed cycles per second of a signal,measured in hertz (Hz).

FREQUENCY HOPPING — The rapid switching (hopping) of radio system frequency for both the receiver and transceiver from frequencyto frequency in apparently random patterns, using a common timing reference.

FSK (Frequency Shift Keying) — A form of modulation in which a digitalsignal shifts the output frequency between discrete values.

GAIN — The ratio of the value of an output parameter, such as power, to its input level. Usually expressed in decibels.

GEOSTATIONARY ORBIT — The speed of a stable satellite orbit dependsupon its distance above the earth. If a satellite is placed in a stable orbit22,300 miles above the equator, it must travel just fast enough to make arotation around the earth in 24 hours. Since that is exactly the same speedthat the earth rotates, a satellite placed in that orbit will hover over thesame spot on earth as they both rotate around together. This is called ageostationary orbit.

GROUND REFLECTED WAVE — The portion of the propagated wavethat is reflected from the surface of the earth between the transmitter and receiver.

GROUND WAVE — A radio wave that is propagated over the earth and ordinarily is affected by the presence of the ground.

HF (High Frequency) — Normally, the band from 3 to 30 MHz. In practice,the lower end of the HF band extends to 1.6 MHz.

Hz (Hertz) — Basic unit for frequency.

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DIPOLE ANTENNA — A versatile antenna that is usually a wire fed atthe center of its length. Its orientation provides either horizontal or vertical polarization.

DIRECT WAVES — Travel in straight line, becoming weaker as distance increases.

DIRECTIONAL ANTENNA — An antenna that has greater gain in one ormore directions.

DOWNLINK — Radio paths back from a satellite.

DSP (Digital Signal Processing) — A recently developed technology that allows software to control digital electronic circuitry.

DUCTING — An effect where radio waves can bend between air layers ofdifferent densities

EMI (Electromagnetic Interference) — An electromagnetic disturbance thatdegrades communications performance. Synonym: Radio FrequencyInterference (RFI).

ENCRYPTION — Process of translating information into an apparently random message.

ERP (Effective Radiated Power) — Equivalent power transmitted to the atmosphere, which is the product of the transmitter power output multiplied by the gain of the antenna.

EXCITER — The part of the transmitter that generates the modulatedsignal for a radio transmitter.

FADING — The variation of the amplitude and/or phase of a received signal due to changes in the propagation path with time.

FEC (Forward Error Correction) — A system of error control for data transmission whereby the receiver can correct any code block that containsfewer than a fixed number of bits in error.

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IF (Intermediate Frequency) — A frequency used within equipment as anintermediate step in transmitting or receiving.

IMPEDANCE — Opposition to current flow of a complex combinationof resistance and reactance. Reactance is the opposition to AC current flowby a capacitor or an inductor. An ideal antenna coupler will act so as tocancel the reactive component of antenna impedance, i.e., by providing anequal inductive reactance if the antenna has a capacitive reactance or anequal capacitive reactance if the antenna presents an inductive reactance.

INTERLEAVING — A technique that increases the effectiveness of FECcodes by randomizing the distribution of errors in communication channelscharacterized by error bursts.

IONOSPHERE — A region of electrically charged particles or gases in theearth’s atmosphere extending from 50 to 600 kilometers (approximately 30 to 375 miles) above the earth’s surface.

ISB (Independent Sideband) — Double sideband transmission in which the information carried by each sideband is different.

JAMMING — Deliberate interference that results from transmission on operating frequencies with the intent to disrupt communications.

KEK (Key Encryption Key) — Used in digital encryption.

KEY — A variable that changes the mathematical algorithm in cryptography.

KEY GENERATOR — A device or process that generates the variable fora cryptographic encoding system.

LF (Low Frequency) — The frequency range from 30 to 300 kHz.

LNA — Low noise receive amplifier.

LOBE — Area of strong radiation

LOS (Line of Sight) — A term that refers to radio signal propagation in astraight line from the transmitter to a receiver without refraction; generallyextends to the visible horizon.

LPD (Low Probability of Detection) — Techniques for minimizing the probability that the transmitted signal is detected by an unauthorized party.

LSB (Lower Sideband) — The difference in frequency between the AMcarrier signal and the modulation signal.

LUF (Lowest Usable Frequency) — The lowest frequency in the HF band at which the received field intensity is sufficient to provide the required signal-to-noise ratio.

MAIN LOBE — In an antenna radiation pattern, the lobe containing thedirection of maximum radiation intensity.

M-ARY PSK (M-ary Phase Shift Keying) — A method of increasing thedata rate of radio transmissions. “M” refers to the number of phases used in the modulation scheme.

MODEM (MOdulator-DEModulator) — A device that modulates anddemodulates signals. The modem converts digital signals into analog formfor transmitting and converts the received analog signals into digital form.

MODULATION — The process, or result of the process, of varying a characteristic of a carrier in accordance with a signal from an information source.

MUF (Maximum Usable Frequency) — The upper limit for the frequenciesused at a specified time for radio transmission between two points viaionospheric propagation.

MULTIBAND — Military radios that combine VHF and UHF, HF and VHF,or HF-VHF-UHF capabilities.

MULTIPATH — The propagation phenomenon that results in radio signalsreaching the receiving antenna by two or more paths.

MULTIPATH SPREAD — The range of timed differences that it takes for radio signals to reach the receiving antenna when they arrive from severalroutes, which may include one or more sky wave paths and/or a ground-wave path. The effect of multipath spread is minimized by selecting a frequency as close as possible to the MUF.

RADIATION PATTERN — Pattern determined by an antenna’s design and strongly influenced by its location with respect to the ground.Radiation patterns are frequency dependent.

RAU — Remote Access Unit.

REFRACTION — The bending of a radio wave as it passes obliquely from one medium to another with different indices of refraction.

SATCOM — Satellite Communications.

SCRAMBLING — A COMSEC technique that involves separating thevoice signal into a number of bands, shifting each band to a different audio frequency range, and combining the resulting bands into a composite audio output that modulates the transmitter.

SERIAL TONE MODEM — Carries digital information on a singleaudio tone.

SHORT WAVE — Radio frequencies above 3 MHz.

SIDEBAND — The spectral energy, distributed above or below a carrier, resulting from a modulation process.

SKY WAVE — A radio wave that is reflected by the ionosphere.

SMC (Satellite Management Center) — Regulates and assigns the satelliteresources to users.

SNR (Signal-to-Noise Ratio) — The ratio of the power in the desired signalto that of noise in a specified bandwidth.

SOFT-DECISION DECODING — An error-correction technique where agroup of detected symbols that retain their analog character are comparedagainst the set of possible transmitted code words. A weighing factor isapplied to each symbol in the code word before a decision is made aboutwhich code word was transmitted.

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NULL — Area of weak radiation

OHM — Unit of measurement of resistance.

OMNIDIRECTIONAL ANTENNA — An antenna whose pattern is non-directional in azimuth.

ON-OFF KEYING — Turning the carrier on or off with telegraph key(Morse code). Same as CW.

OTAR (Over-The-Air-Rekeying) — This technique developed by Harris eliminates the need for manual loading of encryption keys and provides a more secure method of key management.

PARALLEL TONE MODEM — Carries information on simultaneousaudio tones, where each tone is modulated at a low-keying rate.

PHASE — In a periodic process such as a radio wave, any possible distinguishable state of the wave.

PICKET FENCING — Form of multipathing common to vehicularmounted radios

POLARIZATION — The orientation of a wave relative to a referenceplane. Usually expressed as horizontal or vertical in radio wave terminology.

PREAMBLE — A known sequence of bits sent at the start of a message,which the receiver uses to synchronize to its internal clock.

PROPAGATION — The movement of radio frequency energy through the atmosphere.

PSK (Phase Shift Keying) — PSK is similar to FSK except that it is the phase of the carrier rather than the frequency that is modulated.

PUBLIC KEY CRYPTOGRAPHY — A type of key management systemused in the commercial sector. Under this standard, each user generatestwo keys, a public key and a private key. The strength of such a system lies in the difficulty of deriving the private key from the public key.

UPLINK — Radio paths up to a satellite.

USB (Upper Sideband) — The information-carrying band and is thefrequency produced by adding the carrier frequency and the modulatingfrequency.

VERTICAL WHIP ANTENNA — An omnidirectional antenna that has low take-off angles and vertical polarity.

VHF (Very High Frequency) — The portion of the radio spectrum from approximately 30 MHz to 300 MHz.

VOCODER — A device that converts sounds into a data stream that can be sent over a radio channel. Short for voice coder-decoder.

WAVELENGTH — Distance between point on wave to correspondingpoint on adjacent wave.

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SSB (Single Sideband) — A modulation technique in which the carrier and one sideband (upper or lower) are suppressed so that all power is concentrated in the other sideband.

SURFACE WAVES — Travel along the surface of the earth and may reach beyond the horizon.

SYMMETRIC KEY SYSTEM — A key management system in which the same key encrypts and decrypts data.

SYNCHRONOUS — A form of data communications that uses a preamble to alert the data receiver that a message is coming and to allow it to synchronize to an internal bit clock.

TACSAT — Tactical Satellite.

TAKE-OFF ANGLE — The angle between the axis of the main lobe of an antenna pattern and the horizontal plane at the transmitting antenna.

TCM (Trellis Coded Modulation) — A coding technique that providesmaximum data rate capability to PSK data streams by improving the noise margin.

TEK (Traffic Encryption Key) — Used in digital encryption.

TIU — Telephone Interface Unit.

TRAFFIC — The information moved over a communications channel.

TRANSCEIVER — Equipment using common circuits in order to providetransmitting and receiving capability.

TRANSEC (Transmission Security) — Techniques that prevent signal detection or jamming of the transmission path.

UHF (Ultra High Frequency) — The portion of the radio spectrum from300 MHz to 3 GHz.